When a vehicle is stationary on an uphill slope and is to start driving again without rolling backward, it is usual for the driver to hold the vehicle stationary using the handbrake and the footbrake. When the driver is to start driving, he or she then moves the foot from the brake pedal to the accelerator pedal. In order that the vehicle does not start rolling backward, the driver holds the vehicle stationary using the handbrake. The driver then releases the handbrake gently at the same time as applying the accelerator so that the vehicle does not roll backward. This is especially the case for manually shifted vehicles where the left foot operates the clutch.
Many different systems have been proposed for facilitating starting a vehicle which is on an uphill slope. These systems usually go by the name of “hill holder”. They function by the driver, often using a switch, activating the hill start function when the vehicle is stationary. The system then holds the vehicle stationary using the service brake of the vehicle until a predetermined state occurs, for example the driver accelerates, the driver lets the clutch pedal up, the torque of the engine has a certain value, the vehicle rolls forward and the like. The system then releases the brake force.
Known hill start auxiliary systems for vehicles with manual gearboxes are described in such patents as U.S. Pat. No. 6,009,984, U.S. Pat. No. 5,452,946, U.S. Pat. No. 4,676,354, EP 523 338-A2 and EP 507 466-A1. It is a common feature of all of these systems to require one or more extra sensors which detect states in the drive line of the vehicle. Several of the systems have an inclination sensor which senses whether the vehicle is on an uphill gradient. Also included are sensors for sensing the state of the brake pedal and the accelerator pedal, the torque of the engine, the total weight of the vehicle and the like. These systems often function well for the conditions for which they are designed. As the systems are complex, it is difficult or impossible for the driver to know precisely how a certain system functions in a given situation. This may to occasions in which the system functions in a way that is not expected by the driver and could result in near accidents. Moreover, the great complexity of the systems described means that the reliability is lower than for a system that does not require a large number of external components. As an example, a fault on one of the included components can result in the system becoming unserviceable. Another disadvantage of the described systems is that the extra sensors add unnecessary cost to the selling price of the vehicle.
Another problem with the proposed systems is that it is possible to use them as a parking brake. As the vehicle is held stationary by the service brake, this means that the driver can activate the system when parking the vehicle as well, for example when stopping the vehicle for only a relatively short time such as when loading or unloading, or for launch. It is often easier to activate the system than to use the parking brake control. This also means that the driver can as a matter of routine become accustomed to using the system as a parking brake. If something should happen when the vehicle is parked in this way, the vehicle may lose braking capacity altogether. The vehicle may, for example, start to roll in an uncontrolled manner with serious damage as a consequence. This may happen if, for example, the driver is delayed and/or the brake system loses pressure more quickly than expected, for example if a component is slightly leaky.
Another known system is described in U.S. Pat. No. 5,820,515 that is intended for an automatic gearbox. As in the systems mentioned above, this system also requires one or more sensors that detect the states in the drive line of the vehicle, and an inclination sensor which senses whether the vehicle is on an uphill gradient. Also included are sensors or calculation models for estimating the state of the brake pedal and the accelerator pedal, the torque of the engine, the total weight of the vehicle and the like. A disadvantage of this system is the relatively poor efficiency of the torque converter included in the transmission.
The problems mentioned above can be solved by using a stage-geared automated gearbox with an automatic disk clutch, provided with a control system which coordinates the functioning of the service brake, the engine and the disk clutch of the automated gearbox.
Solutions configured according to these teachings of the invention prevent the vehicle from rolling in the wrong direction during a starting sequence, which reduces the loading in the clutch and moreover minimizes the risk of accidents.
The solution according to the invention also prevents the hill start system from being used as a parking brake. In the invention, only sensors located in the gearbox are used, for which reason no unnecessary cost is added to the selling price of the vehicle. The solution is moreover robust in relation to faults in component sensors as the necessary signals can in such a case be calculated in other ways. The solution functions in an intuitive way for the driver, which means that the driver is not surprised by the performance of the system with the result that a near-accident occurs.